Known dry-type transformers have advantages over oil-immersed units. These advantages can include, for example, a reduced risk of fire and explosion, increased environmental friendliness, maintenance free, and a capability to be installed closer to the consumption point.
Delta type transformer cores with different cross-sectional shapes have been proposed as an alternative to the known stacked core design with coplanar limbs, as they exhibit several comparative advantages: The no-load losses are lower, size and weight can be smaller, the inrush current is lower, and total harmonic distortion is lower. A Chinese company, Haihong Transformer, for example, produces delta core transformers including three wound core rings with approximately semi-circular cross-sections each. Another implementation of a wound delta core is provided by the Swedish company Hexaformer AB. The name Hexaformer hereby comes from the fact that the cross-sections of the limbs form regular hexagons, while the arrangement of the limbs still results in a rotational symmetric delta shaped core. WO 2006/056057A1 discloses an enclosureless delta shaped transformer with a cooling channel provided between the 3 core limbs in the centre of the transformer. Heat is removed from the transformer by air blown inside the channel by fans paced at the ends of the channel.
In known implementations, SF6 is used as an insulating gas. Due to the good dielectric and cooling capabilities of SF6, even high end distribution transformers with rated voltages and powers up to 170 kV and 60 MVA can be manufactured with moderate SF6 pressures, for example, equal to or lower than 2 bar.
However, due to the absence of oil, dry-type transformers are more demanding with respect to dielectric and thermal design and consequently they are larger and heavier than the corresponding oil-immersed transformers. DE4029097A1 discloses a delta shaped transformer in a gas insulated cylindrical housing. Cooling channels are formed in each corner of the delta shaped core between two adjacent core limbs. As a result, gas circulation reaches the transformer housing.
When the rated electrical loads of dry transformers are increased, cooling becomes an increasingly important subject, as there is no liquid—as in the case of oil-immersed units—which can be used as a cooling medium. Rather, the insulating gas can also serve for transporting produced heat to an outside of the transformer. However, gas can have a much smaller ability to transport heat than the same volume of liquid. Thus, the heat transport to an outside of a gas insulated delta shaped transformer can include more attention in the design phase than with a known oil-immersed type.
Even more so, due to the rotational symmetry of the delta shaped transformers, the high voltage coil outer walls adjacent to the transformer centerline can have nearly the same temperature. For this reason, the delta shape arrangement of the transformer can be characterized by a limited radiative heat exchange between the wall parts facing its center. Rather, the heat emitted from a coil towards the other two coils is absorbed by those, which in summary effectively reduces the heat emitted from the transformer to an outside, for example when compared with a design with three coils arranged in parallel in a plane (coplanar design).
In view of the above, designs for gas insulated delta shaped transformers should deliver improved cooling capabilities.